tint/test-runner: Split expectations for FXC and DXC

Change tint's `--fxc` flag to take the path of the FXC compiler DLL.
Have tint attempt to validate with both FXC and DXC if `--validate` is
passed.

Fix the 'dirsWithNoPassExpectations' logic which looks like it got
broken with the tint -> dawn merge. It also incorrectly applied
filepath.FromSlash() on windows.

Change-Id: I0f46aa5c21bc48a2abc48402c41f846aff4a8633
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/96800
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@chromium.org>

test/tint/var/initialization/function/array/array_i32.wgsl.expected.fxc.hlsl

@@ -0,0 +1,8 @@
+[numthreads(1, 1, 1)]
+void main() {
+  int zero[2][3] = (int[2][3])0;
+  const int tint_symbol[3] = {1, 2, 3};
+  const int tint_symbol_1[3] = {4, 5, 6};
+  int init[2][3] = {tint_symbol, tint_symbol_1};
+  return;
+}

test/tint/var/initialization/function/array/i32.wgsl.expected.fxc.hlsl

@@ -0,0 +1,6 @@
+[numthreads(1, 1, 1)]
+void main() {
+  int zero[3] = (int[3])0;
+  int init[3] = {1, 2, 3};
+  return;
+}